.net从服务端下载文件（可以断点续传）

public void DownFile(string guid)
        {
            var fileTransfer = new FileTransfer();
            var directoryPath = Path.Combine(InitInfo.Config_GarbagePath, "FileImport", guid);
            fileTransfer.ChunkTransfer(System.Web.HttpContext.Current, GetDownFilePath(directoryPath));
            HttpContext.Response.Flush();

            //删除临时文件
            //   Utility.DeleteDirectory(directoryPath);
        }

 public bool ChunkTransfer(HttpContext httpContext, string filePathName)
        {
            bool result = false;

            if (!System.IO.File.Exists(filePathName))
            {
                httpContext.Response.StatusCode = 404;
                return false;
            }
            long startPostion = 0;
            long endPostion = 0;
            string fileName = Path.GetFileName(filePathName);
            using (FileStream fileStream = new FileStream(filePathName, FileMode.Open, FileAccess.Read, FileShare.Read))
            {
                using (BinaryReader br = new BinaryReader(fileStream))
                {
                    long fileLength = fileStream.Length;
                    string lastUpdateTime = System.IO.File.GetLastWriteTimeUtc(filePathName).ToString("r");
                    string eTag = HttpUtility.UrlEncode(fileName, Encoding.UTF8) + lastUpdateTime;//恢复下载时提取请求头; 
                    if (httpContext.Request.Headers["If-Range"] != null) 
                    {
                        if (httpContext.Request.Headers["If-Range"].Replace(""", "") != eTag)
                        {//文件修改过  
                            httpContext.Response.StatusCode = 412;//预处理失败  
                            return false;
                        }
                    }

                    httpContext.Response.Clear();
                    httpContext.Response.Buffer = false;
                    httpContext.Response.AddHeader("Accept-Ranges", "bytes");
                    httpContext.Response.AppendHeader("ETag", """ + eTag + """);
                    httpContext.Response.AppendHeader("Last-Modified", lastUpdateTime);//把最后修改日期写入响应                  
                    httpContext.Response.ContentType = "application/octet-stream";
                    if (httpContext.Request.UserAgent.IndexOf("MSIE") > -1 || (httpContext.Request.UserAgent.IndexOf("like Gecko") > -1))
                    {
                        fileName = HttpUtility.UrlEncode(fileName, Encoding.UTF8).Replace("+", "%20");
                    }

                    if (httpContext.Request.UserAgent.ToLower().IndexOf("firefox") > -1)
                    {
                        httpContext.Response.AddHeader("Content-Disposition", "attachment;filename="" + fileName + """);
                    }
                    else
                    {
                        httpContext.Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
                    }
                    httpContext.Response.AddHeader("Connection", "Keep-Alive");
                    httpContext.Response.ContentEncoding = Encoding.UTF8;
                    if (httpContext.Request.Headers["Range"] != null)//续传
                    {
                        httpContext.Response.StatusCode = 206;//续传标识
                        string[] range = httpContext.Request.Headers["Range"].Split(new char[] { '=', '-' });
                        startPostion = long.Parse(range[1]);//已经下载的字节数
                        if (startPostion < 0 || startPostion >= fileLength)
                        {
                            return false;
                        }
                        if (string.IsNullOrEmpty(range[2]))//只指定请求文件起始
                        {
                            httpContext.Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startPostion, fileLength - 1, fileLength));
                            httpContext.Response.AddHeader("Content-Length", (fileLength - startPostion).ToString());
                        }
                        else//指定请求文件范围
                        {
                            endPostion = long.Parse(range[2]);
                            httpContext.Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startPostion, endPostion - startPostion - 1, fileLength));
                            httpContext.Response.AddHeader("Content-Length", (endPostion - startPostion).ToString());
                        }
                    }
                    else
                    {//非续传
                        httpContext.Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startPostion, fileLength - 1, fileLength));
                        httpContext.Response.AddHeader("Content-Length", (fileLength - startPostion).ToString());
                    }

                    br.BaseStream.Seek(startPostion, SeekOrigin.Begin);
                    long maxCount = (long)Math.Ceiling((fileLength - startPostion + 0.0) / TransferBuffer);//分块下载，剩余部分可分成的块数  
                    for (long i = 0; i < maxCount && httpContext.Response.IsClientConnected; i++)
                    {
                        httpContext.Response.BinaryWrite(br.ReadBytes(TransferBuffer));
                        httpContext.Response.Flush();
                        if (TransferSleep > 0)
                        {
                            Thread.Sleep(TransferSleep);
                        }
                    }

                    result = true;

                }
            }


            return result;
        }